1. Field of the Invention
The subject invention relates to methods and apparatus for advancing tape, including by way of example magnetic and other recording tape and various tape, web and sheet-like materials. The subject invention more specifically relates to magnetic and other tape recording and reproducing methods and apparatus and to magnetic tape transports.
2. Prior-Art Statement
In the context of methods and apparatus for advancing a magnetic recording or other tape, it is frequently important to achieve rapid tape speed changes at minimum energy expenditure. Especially in bidirectional tape drives, it is frequently essential that the tape reverse its direction of travel as rapidly as possible without overloading the system. Similar desiderata occur in the context of other high-performance systems requiring high tape acceleration and deceleration capability.
In this respect, the tape drive capstan, as a traditional tape drive element, brings into play countervailing considerations. In particular, decreasing the capstan diameter would have the advantage of a relative inertia reduction, thereby removing a major impediment to rapid tape acceleration and deceleration.
Increasing the capstan diameter, on the other hand, would yield the advantages of a larger relative tape speed, enabling more rapid acceleration per revolution, and of a larger capstan-to-tape interface for better tape friction.
Considering at the moment only the countervailing capstan inertia and tape velocity consideration, a point can generally be found at which the capstan diameter presents a tradeoff between inertia and tape velocity in the sense of a minimum energy expenditure.
In this context, reference may be had to U.S. Pat. No. 3,443,039, by P. A. Bygnes, wherein a capstan is closely straddled by a tape guide block. Since this arrangement in that prior-art proposal removes the tape from contact with all but two small portions of the capstan periphery, pinch rollers have to be provided which clamp the tape to the capstan in the two nip or contact areas for the transfer of motive power to the tape.
Unfortunately, true tape speed in a pinch roller system is not equal to the surface speed of the capstan, since pinch rollers typically are of a compliant kind, being applied to the tape at the capstan with a sufficient amount of pressure to provide the requisite friction at the nip for the desired tape advance. In particular, the pinch roller pressure causes an indentation in the compliant pinch roller whereby the effective radius of the roller is reduced, causing the tape to experience a speed variation as between its entry and exit from the nip. As a result, the tape speed is practically determined by the pinch roller, rather than by the capstan. Pinch rollers also add considerable inertia to capstan servo systems, thus impeding servo bandwidth, and are a frequent source of trouble in terms of difficulty of alignment, exposure to dust and other contaminants and subjection to bearing fatigue and further durability limiting factors.
These negative aspects have led to the development of a pure friction drive known under the term "capstan wrapping", wherein the tape extends around part of the capstan at a sufficient wrap angle to generate the requisite drag force on the tape. As an example of an advanced design providing sufficient wrap angles even when the tape is lifted from the capstan for recording and playback purposes, reference may be had to U.S. Pat. No. 4,054,929, by A. Levy, assigned to the subject assignee and herewith incorporated by reference herein.
That Levy patent shows how capstans may be provided with several circumferential grooves to reduce the inclusion of friction reducing air pockets between tape and capstan. Grooved capstans are also apparent from U.S. Pat. Nos. 3,143,267, by A. R. Maxey, 3,614,338, by P. W. Bogels, 3,840,895, by M. Kubo, and 4,029,249, by P. Nagel et al.
According to U.S. Pat. No. 4,065,044, by A. Painter et al, a vacuum source to one side of a grooved capstan pulls a length of magnetic tape against the capstan periphery by creating a vacuum within the grooves, thereby frictionally engaging the tape with the capstan along the particular wrap angle. Even though that proposal also contemplates reversing the vacuum supply to an air supply to allow the length of magnetic tape to be blown off the capstan in order to disengage the tape therefrom, it can in practice not reliably be avoided that the tape become engaged with the capstan ahead of or beyond the desired wrap angle.
This is particularly the case in rapidly changing bidirectional drives, wherein the tape may more easily become caught by and entangled on the capstan. U.S. Pat. No. 3,122,295, by R. H. Davison et al recognizes this problem and, in the context of a bidirectional tape drive, continuously supplies air to a grooved capstan to prevent the tape from becoming entangled therewith. In practice, this is a rather drastic measure, since the firm adherence of the tape to the capstan as potentially attainable in vacuum capstan systems could be a very valuable property. In particular, a temporary vacuum attachment of the tape to the capstan would permit a reduction of the effective tape/capstan interface, and thereby a reduction of the capstan size with resulting diminution of capstan inertia. However, as may, for instance, be seen from U.S. Pat. No. 3,688,956, by M. J. Kjos, use of a vacuum capstan may engender other problems. According to Kjos, air is drawn from the capstan surface into a hollow capstan in order to draw the tape firmly into contact with the capstan at two capstan surface areas having a pressurized air tape lifting region located there between. In order to avoid entanglement of the tape with the vacuum capstan, Kjos provides so-called "vacuum blocks" inside the apertured hollow capstan. In practice, such "vacuum blocks" tend to impose design and performance limitations on the system, since the "vacuum blocks" have to be maintained stationary within the hollow rotating capstan. Switching the vacuum on and off from the inside of the capstan also generates abrupt air flow changes externally manifested as noise.
A series of prior-art tape drives uses a hollow-cylindrical capstan having peripheral apertures. The tape is attracted to the capstan within a predetermined arc by withdrawing air into the hollow-cylindrical capstan through peripheral apertures located at the time within that arc, and further from the hollow-cylindrical capstan through peripheral apertures located at the time outside of that arc. The tape is guided to a beginning of the mentioned arc and, upon being attracted to the capstan by the mentioned air withdrawal, is driven through that arc by the capstan drive motor. The tape is removed from the capstan at the end of the mentioned arc. Such tape drives are shown in the publication DC MOTORS SPEED CONTROLS SERVO SYSTEMS, an Engineering Handbook by Electro-Craft Corporation, of Hopkins, Minn. 55343, particularly pp. 7-30 to 7-37.
In practice, such prior-art tape drives display relatively abrupt air flow changes at the beginning and the end of the mentioned arc.
Two further references located in a novelty search, namely U.S. Pat. No. 3,560,946, by G. J. Ehalt et al, wherein vacuum shoes are employed for holding record members to a rotor within a constant tolerance, and U.S. Pat. No. 3,872,507, by K. Sano et al, wherein air recesses are formed in the outer peripheral surface of a drum or in spacers provided along the length of a magnetic head, do not appear to suggest a solution to the subject problem.